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In polymyositis, inflammation of the muscles causes muscle weakness and wasting, particularly around the shoulders and pelvis. If a rash also occurs, the disorder is known as dermatomyositis. Both conditions are rare autoimmune disorders in which tissues are attacked by the body’s own antibodies. The cause of this abnormal immune reaction is not known, although its onset may be triggered by a viral infection. In adults, these diseases may be associated with cancer. They can also occur in association with other autoimmune disorders, such as systemic lupus erythematosus. Polymyositis and dermatomyositis sometimes run in families, which suggests that genetic factors may be involved. Both disorders are more common in females and tend to occur in childhood and middle age.

What are the symptoms?

The symptoms of polymyositis and dermatomyositis may develop rapidly, particularly in children; in adults, the symptoms usually develop over several weeks. Symptoms may include:

Weakness of affected muscles, leading, for example, to difficulty raising the arms or getting up from a sitting or squatting position.

Painful, swollen joints.

Tiredness.

Difficulty swallowing if the muscles of the throat are affected.

Shortness of breath if the heart or chest muscles are involved.

In dermatomyositis, the symptoms may be preceded, accompanied, or followed by:

A red rash, often on the face, chest, or backs of the hands over the knuckles.

Swollen, reddish-purple eyelids.

About half of all children who have dermatomyositis develop skin ulcers.

What might be done?

If polymyositis or dermatomyositis is suspected, your doctor may arrange for blood tests to investigate inflammation and to look for antibodies that are specific to these conditions. You may also have electromyography (see Nerve and muscle electrical tests) to measure electrical activity in the muscle, and a muscle biopsy, in which a sample of muscle tissue is removed for examination. An ECG may be carried out to check for involvement of the heart muscle. You may also have a chest X-ray and other tests to exclude the possibility of an underlying cancer.

There is no cure for polymyositis or dermatomyositis. However, the symptoms can usually be controlled. You will probably be prescribed high doses of corticosteroids to help to reduce the inflammation. If there is no marked improvement, an immunosuppressant drug or a drug that is normally used to treat cancer (see Anticancer drugs), may be given to slow the progression of the disorder. If dermatomyositis does not respond to these drugs, injections of immunoglobulins (antibodies) may be given. Physiotherapy may help to prevent muscle stiffness and restore strength.

What is the prognosis?

The outlook for both polymyositis and dermatomyositis is better in children than in adults. About 7 in 10 affected children recover completely in 2 years. Adults with polymyositis or dermatomyositis may need corticosteroid drugs for several years. In people who have cancer, the autoimmune disorder may improve if the cancer can be treated.